1. Field of the Invention
This invention relates to dynamometers for simulating the inertia and road load forces encountered by motor vehicles under anticipated driving conditions and more particularly to a dynamometer which eliminates the need to compensate for certain unmeasured equipment parasitic losses inherent in the dynamometer's operation.
2. Description of the Prior Art
Dynamometers are often used in testing motor vehicles such as automobiles, trucks, motorcycles etc. where in situ operation is desired. Since the test vehicles are not moving over a road bed, the dynamometer must simulate certain forces normally associated with actual vehicle operation. These parameters include forces associated with inertial forces (related to the mass or weight of the vehicle) and road load forces (related to the velocity of the vehicle). The vehicle engine (or its braking system) must overcome inertial forces in order to accelerate or decelerate the vehicle. In addition, the engine must overcome breakaway frictional and rolling frictional forces (i.e., road/tire friction) as well as windage forces (i.e., drag forces caused by air passing over the vehicle). These latter forces are commonly referred to as road load (RL) forces and may be represented by the formula: EQU RL=A+BV+CV.sup.n +D
where A, B and C represent the effects of breakaway force, rolling friction and windage, V represents velocity, D represents the grade of the slope and n is the exponent to which the velocity V is raised.
The purpose of the dynamometer is to impose those forces on the vehicle which the vehicle would incur during actual operation on a road. Such dynamometers include a roll (or a pair of rolls) for engaging the driven wheel (e.g., motorcycle) or wheels (e.g., automobile) of the vehicle being tested. The roll or rolls are supported by a shaft journaled in bearings mounted on a frame.
Typically a power absorber such as a friction brake, eddy current brake or hydrokinetic brake carried by the same or a different frame is coupled to the roll for absorbing power from the roll which in turn applies a retarding force to the surface of the vehicle wheel (e.g., tire) to simulate the road load forces. Inertial forces can also be simulated by such power absorbers during acceleration but not during deceleration since such units absorb but do not supply power. Generally where such power absorbers ("power absorbing units") are used, the inertial forces are simulated by selectively coupling the roll to one or more mechanical flywheels. The combined inertia of the flywheels and the roll (as well as the absorber) exert force on the vehicle wheels proportional to the acceleration (or deceleration) of the vehicle wheels. Thus, the engine is required to expend as much power in accelerating the roll as it does in overcoming the vehicle inertia during actual road acceleration. The use of flywheels alone to accurately simulate inertia for a variety of vehicles is limited by the number and size of flywheels available. The larger the number of flywheels the greater the cost and complexity of the dynamometer.
Electric motors have the capability of supplying as well as absorbing power and for this reason have been used to simulate both vehicle inertia and road load forces. One or several flywheels may be used in conjunction with such motors ("power supplying and absorbing units") to minimize the size of the motor required and therefore the cost of the resulting dynamometer, its installation and operation. Vehicle speed and acceleration may be computed from the formula: ##EQU1## where V.sub.1 = computed velocity at time t.sub.1, V.sub.0 = the velocity at time t.sub.0, F = the measured force at the wheel/roll interface, and I = the simulated vehicle inertia and RL = road load force.
To control a power supplying and/or absorbing unit accurately, it is necessary, therefore, to measure V (representative of the vehicle velocity) and F (representative of the force at the wheel/roll interface). A dynamometer controller responsive to signals (e.g., electrical) representing V and F and the inertia and road load forces to be simulated supplies the appropriate control signals to the power supplying and/or absorbing unit.
The rotational velocity of the roll is representative of V and can be accurately measured by coupling a speed encoder of the optical or magnetic pulse type to the dynamometer roll. However, there is no force measuring device which as a practical matter, can be placed between the rotating vehicle wheel and the roll. As a compromise, typical prior art dynamometers have placed a force measuring device or transducer ("load cell") either at the output of the power supplying and/or absorbing unit or between the flywheel assembly and the shaft connecting the flywheels to the roll. In either case, there are bearing friction and windage losses generated by the roll and/or flywheels which are not measured by the load cell. Such losses are commonly referred to as parasitic losses and must be compensated for in order to provide an accurate control signal for the dynamometer.
A parasitic loss profile or curve of the lost force at the roll surface versus roll speed for the roll (and any other components such as flywheels located between the load cell and the roll shaft) can be computed by measuring the force required to maintain the roll at several selected (e.g., three) speeds. Such a loss profile can also be calculated by using the actual inertia of the roll system and allowing the roll to coast down from a high speed while measuring the change of roll speed at selected points on the speed curve. A signal representative of the parasitic losses can then subtracted from the control signal fed to the dynamometer. However, the accuracy of such prior art dynamometers is limited by the fact that parasitic losses and particularly frictional losses can vary with temperature, wear and other factors.
U.S. Pat. No. 4,324,133, assigned to the assignee of this application, describes a torque measuring device for dynamometers in which the roll is mounted on linkage pivotally attached to a fixed frame. A power absorber is also mounted on the fixed frame and coupled to the roll through a shaft and gear box. A load cell is connected between the linkage and the fixed frame. While the load cell in such an arrangement will sense the parasitic losses which occur within the roll bearings and gear box, it will not account for the force associated with the slippage between the gear box and the spline which connects the gear box to the absorber.
Thus, there is a need for a dynamometer which is arranged so that the forces accompanying the parasitic losses of the roll, power supplying and/or absorbing unit and the coupling therebetween is included in the value measured by the force sensing means to thereby eliminate the need to compensate for such losses.